First observation of the rare $B^{+}to D^{+} K^{+} pi^{-}$ decay

The $B^{+}to D^{+} K^{+} pi^{-}$ decay is observed in a data sample corresponding to $3.0,{rm fb}^{-1}$ of $pp$ collision data recorded by the LHCb experiment during 2011 and 2012. The signal significance is $8,sigma$ and the branching fraction is measured to be ${cal B}left(B^{+}to D^{+} K^{+} pi^{-}right) = (5.31 pm 0.90 pm 0.48 pm 0.35)times 10^{-6}$, where the uncertainties are statistical, systematic and due to the normalisation mode $B^{+}to D^{-} K^{+} pi^{+}$, respectively. The Dalitz plot appears to be dominated by broad structures. Angular distributions are exploited to search for quasi-two-body contributions from $B^{+}to D^{*}_{2}(2460)^{0}K^{+}$ and $B^{+}to D^{+} K^{*}(892)^{0}$ decays. No significant signals are observed and upper limits are set on their branching fractions.